1. Field of the Invention
The present invention relates to a reluctance motor used for driving a compressor of such as an air conditioner or a refrigerator.
2. Description of the Related Art
As the structure of a typical reluctance motor, one shown in FIG. 18 is conventionally well known. Referring to FIG. 18, a description will be given hereafter of the structure of the conventional reluctance motor.
FIG. 18 is a cross-sectional view illustrating a general reluctance motor, in which reference numeral 1 denotes a rotor shaft; 2 denotes a rotor; 3a, 3b, 3c, and 3d denote pole protrusions of the rotor; 4 denotes a stator; 5a, 5b, . . . , 5f denote pole protrusions of the stator; 6 denotes a stator winding; 7 denotes a gap between the stator 4 and the rotor 2; 12 denotes a groove provided in the stator 4; and 13 denotes an insulating portion surrounding the groove 12.
The rotor 2 is formed by laminated steel plates which are laminated in the direction of the rotating shaft 1, and the rotor 2 has around the rotor shaft 1 a plurality of, e.g., four, pole protrusions 3a, 3b, 3c, and 3d arranged at equal intervals and having equal shapes. The stator 4 is formed by laminated steel plates which are laminated in the direction of the rotating shaft 1 in the same way as the rotor 2. The stator 4 is arranged around the rotor 2 with the gap 7 therebetween, and has a plurality of, e.g., six, pole protrusions 5a, 5b, . . . , 5f arranged at equal intervals and having equal shapes. The stator winding 6 is wound around each of the pole protrusions 5a, 5b, . . . , 5f of the stator. Generally, each of the pole protrusions 5a, 5b, . . . , 5f of the stator has a straight-sided pole profile in which its lateral faces are parallel with each other.
Next, referring to FIG. 19, a description will be given of the basic principle of the driving of the reluctance motor having the above-described configuration. In the drawing, STEP 1, STEP 2, and STEP 3 show states in which the relative positional relationships between the rotor 2 and the stator 4 differ. STEP 1 shows a state in which the pole protrusion 5a of the stator and the pole protrusion 3a of the rotor are remote from each other, while STEP 2 and STEP 3 show states in which the pole protrusion 5a of the stator and the pole protrusion 3a of the rotor are close to each other. If the winding of a phase A is excited in the direction shown in the drawing when the pole protrusion 5a of the stator and the pole protrusion 3a of the rotor are located in the state shown in STEP 1, the magnetic flux is headed from a corner of the tooth of the pole protrusion 5a of the stator toward a corner of the tooth of the pole protrusion 3a of the rotor, passes along the path indicated by the broken line in the drawing, and flows while being curved. At this time, since the magnetic line of force is in an unstable state, and magnetic attraction acts in a magnetically stable direction in which the magnetic flux flows straightly, the rotor 2 moves in the direction of arrows.
When the rotor 2 reaches the position of STEP 2 which is magnetically most stable, the magnetic attraction ceases to act in the circumferential direction and acts only in the radial direction, so that torque does not occur. Accordingly, by changing over the energizing phase from phase A to phase B, the pole protrusion 5f of the stator and the pole protrusion 3d of the rotor are set in a state of being spaced apart from each other, and produce the magnetically unstable state again, thereby producing a torque. For this reason, the rotor 2 moves in the direction of the arrow shown in STEP 3.
In the above-described manner, the rotor 2 can be rotatively driven by consecutively changing over the energizing phases of the windings in correspondence with the positions of the pole protrusions 3 of the rotor 3. At this time, if it is assumed that the winding current is i, the number of turns is N, the winding inductance is L, and the position of the rotor is .theta., then the torque T which is generated in the range where magnetic saturation does not occur can be expressed by Formula (1): EQU T=1/2.multidot.(N.multidot.i).sup.2 .multidot.dL/d.theta. (1)
Namely, torque is proportional to the square of the winding current and the change in the winding inductance with respect to the position of the rotor. In particular, the change in the inductance is a term which is largely ascribable to the tooth profiles of the pole protrusions 3 of the rotor and the pole protrusions 5 of the stator. That is, to make the torque large, it is desirable to enlarge the salient pole ratio (Lmax/Lmin), i.e., the ratio between the inductance at the position where the teeth of the pole protrusion 3 of the rotor and the pole protrusion 5 of the stator are aligned with each other with respect to the energizing phase (maximum inductance: Lmax) and the inductance at the position where the teeth of the pole protrusion 3 of the rotor and the pole protrusion 5 of the stator are most remote from each other (minimum inductance: Lmin). For this reason, with the conventional reluctance motor, the tooth profile of each of the pole protrusions 3 and 5 has parallel straight sides from the standpoint of improvement of the salient pole ratio.
In addition, FIGS. 20 and 21 show a variable reluctance motor disclosed in Japanese Utility Model Application Laid-Open No. 65056/1990. FIG. 20 is a side elevational view schematically illustrating the conventional variable reluctance motor, and FIG. 21 is a perspective view, partly in section, of the stator. In the drawings, reference numeral 1 denotes the rotor shaft; 2 denotes the rotor; 3a, 3b, 3c, and 3d denote the pole protrusions of the rotor; 4 denotes the stator; 5a, 5b, and 5c denote the pole protrusions of the stator; 6 denotes the winding; 19a and 19b denote projecting portions; 20 denotes a retaining member; and 21 denotes a retaining portion.
As shown in the drawings, the stator 4 has a plurality of pole protrusions 5 on its inner peripheral side, and the winding 6 is wound around each pole protrusion 5. The rotor 2 is disposed on the inner peripheral side of the stator 4, and the plurality of pole protrusions 3 are provided on its outer peripheral side. The retaining members 20 which are formed from an insulating material are respectively retained in such a manner as to straddle the pole protrusions 5, and abut against the windings 6 wound around the respective pole protrusions, so as to prevent the windings 6 from coming of the pole protrusions 5.
In addition, FIG. 22 is a cross-sectional view schematically illustrating the conventional compressor-driving motor. In the drawing, reference numeral 1 denotes the rotor shaft; 2, the rotor; 4, the stator; 6, the winding; and 22, a slotted portion.
The conventional compressor-driving motor is constituted by an induction motor, a permanent magnet-type motor, or the like, and its rotor has the structure of a round cross section, as shown in FIG. 22. For this reason, in the case where the motor is used as the compressor-driving motor, the outer peripheral portion of the stator 4 is not formed with a completely circular shape, but is provided with the slotted portions 22.
The conventional reluctance motor configured as described above has had the following problems.
With the reluctance motor sown in FIG. 18, since the tooth profile of each pole protrusion 5 of the stator has a parallel straight-sided shape, it is difficult to hold the windings 6, and if an attempt is made to increase the space factor of the windings 6, there occurs a failure such as the slipping off of the windings 6 to the inner peripheral side of the stator 4, thereby hampering the improvement of the space factor. Further, if the space factor is made unduly large, the windings 6 bulge to the inner peripheral side of the stator 4, giving rise to the failure such as that it becomes impossible for the rotor 2 to be incorporated on the inner peripheral side of the stator 4. For this reason, with the conventional stator 4 having the straight sides, it has been inevitable to make the space factor of the windings 6 small. Namely, it has been necessary to decrease the wire diameter of the windings 6 or reduce the number of turns. This results in an increase in the winding resistance and an increase in the winding current, so that there have been problems in that the copper loss increases, and the efficiency declines.
In addition, with the reluctance motor shown in FIGS. 20 and 21, the projecting portions 19a and 19b, which project in the circumferential direction, are provided at inner peripheral end portions of each pole protrusion 5 of the stator so as to prevent the windings 6 from slipping off by means of these projecting portions 19a and 19b, thereby facilitating the holding of the windings 6. These projecting portions 19a and 19b are provided at all the inner peripheral end portions of the pole protrusions 5 of the stator over the entire axial length.
As described above, the reluctance motor has a double salient pole structure in which both the stator 4 and the rotor 2 have pole protrusions, and it has been clarified through experiments and magnetic-field analysis that, to bring out the torque most effectively, in light of the improvement of the salient pole ratio it is desirable to make substantially equal the dimensions of the tooth width of the pole protrusion 5 of the stator and the groove width of the stator 4 and make substantially equal the dimensions of the tooth width of the pole protrusion 5 of the stator and the tooth width of the pole protrusion 3 of the rotor. However, in the structure of the projecting portions 19a and 19b provided over the entire axial length, the tooth width of the pole protrusion 5 of the stator becomes large relative to the groove width of the stator 4, resulting in a decline in the salient pole ratio necessary for effectively bringing out the torque. Consequently, to obtain the same torque, a greater current is conventionally required, causing an increase in copper loss and deteriorating the efficiency.
In addition, the stator of the compressor-driving motor is provided with the slotted portions 22 in the outer peripheral portion of the stator to secure passages for a refrigerant gas, but these slotted portions 22 makes the core back portion of the stator thin. As a result, the magnetic flux density of the core back portion increases, which deteriorates the magnetic characteristics, resulting in an increase in iron loss.